Abstract
Introduction: The medial tibial plateau is composed of two relatively flat facets. An anterior upward sloping “extension facet” (EF) articulates with the medial femoral condyle from 0 to 20° – the stance phase of gait (in Man but not in other mammals). A horizontal “flexion facet” contacts the femur from 20° to full flexion. Anatomical variation in this area might be responsible for the initiation of antero-medial osteoarthritis (AMOA).
This paper reports the angle between the EF and the horizontal (the extension facet angle - EFA) in normal knees and in knees with early AMOA.
Method: MRI reports were searched to identify patients with acute rupture of the ACL on the assumption that they had anatomically normal tibiae (46 males and 18 females) and patients with MRI evidence of early AMOA without bone loss (11 males and 9 females).
A sagittal image at the midpoint of the femoral condyle was used to determine the EFA. Repeat measurements were taken by two observers.
Results: The EFA in normal tibiae is 14 +/− 5° (range 3 – 25°). The angle is unrelated to age. The EFA in individuals with early AMOA is 19 +/− 4° (range 13 – 26°). The difference is highly significant (p< 0.001).
Discussion: There is a wide variation in the EFA in normal knees that is unrelated to age.
There is an association between an increased EFA (ie a steeper EF) and MRI evidence of AMOA. Although a causal link is not proven, we speculate that a steeper angle increases the duration of loading on the EF in stance and tibio-femoral interface shear. This may initiate cartilage breakdown.
Correspondence should be addressed to: Tim Wilton, BASK, c/o BOA, The Royal College of Surgeons, 35–43 Lincoln’s Inn Fields, London WC2A 3PE.
